Power tongs and control system

ABSTRACT

A control system for power tongs comprises two tongs torque limiters, one of which limits tongs torque to a very low (hand-tight) level; the other limits torque at full makeup to, for example, 2000 ft. lb. The hand-tight torque limiter is manually or automatically disabled after the hand-tight level is reached. The system enables the operator to verify that the threads are properly engaged before applying full torque to the connection. The tongs have a very high reduction ratio, so that torque rises very slowly and can be more accurately controlled.

.Iadd.This is a continuation of application Ser. No. 041,061 filed Apr.22, 1987, now abandoned. .Iaddend.

This invention relates generally to torque-limiting control systems forpower tongs used to make up well pipe connections.

BACKGROUND

Well pipe is made up by supporting a lower pipe section ("joint") in thewell and then threading an upper joint onto it by means of afluid-driven power tongs. The pipe assembly is lowered as new joints areadded, down to depths of several miles. Threaded well joint connections,in order to seal properly and to have maximum tensile strength, must beaccurately tighted ("made-up" in the trade) to a design torque ("make-uptorque") specified by the pipe manufacture. The design torque must notbe exceeded, since galling or breakage of the pipe threads may result.This is particularly true with pipe joint materials chosen forconsiderations other than strength, e.g. corrosion resistance andimpermeability. Such materials are not only relatively soft--they can bequite expensive. In one recent case, 1000 joints (each thirty-three feetlong) were removed from a well. Every joint had thread damage due toovertorquing and was considered scrap. This was pipe originally costing$2500 per joint. The importance of controlling the torque applied by thepower tongs to the pipe can thus be appreciated, and in fact it is arequirement on many jobs that a running record of maximum torque at eachjoint be kept. (Various systems exist for making torque records duringmake-up, including applicant's system described in copending applicationSer. Nos. 487,048.Iadd., now U.S. Pat. No. 4,552,041, .Iaddend.and526,611.Iadd., now abandoned.Iaddend..) Despite the existence ofaccurate torque recording systems, improper torquing continues to occur.The industry still seeks a system that will positively prevent threaddamage from overtorquing.

A second consideration is that thread damage can result not only fromovertorquing but also from pipe misalignment. When the hoist supportingthe upper end of a joint undergoes large lateral excursion occasionedperhaps by high winds, misalignment sufficient to cause cross-threadingcan occur. Once the threads are crossed, not much torque is required toruin the threads. If the crossed thread is not detected, a leakyconnection can result even though the proper torque is applied, since inthat instance torque may not be an adequate indicator of sealing force.

The crossed thread problem is aggravated by violent or jerky movement ofthe tongs when power is first applied. The tongs frequently do not worksmoothly--and are hard to control--at very low speeds. Also, the snubline, initially slack, tends to snap tight when power is first applied.These conditions make it difficult to control and/or record torque atthe instant tongs operation begins, so that thread damage can occur evenif a low-level torque limiter is used.

Even if the threads are not crossed, misalignment of the pipes can causebinding of the threads sufficient to produce galling as the pipe isrotated.

I have found that the above problems can be overcome by substantiallyincreasing the overall gear reduction ratio within the tongs, forexample, by a factor of five. The tongs jaw speed is correspondinglyreduced, avoiding the problems of irregular start-up. This speedreduction is advantageously combined with a two-stage torque limitersystem for (a) preventing the application of substantial torque duringthe initial phase of makeup and (b) limiting the maximum torque that thetongs can produce at the final makeup stage.

This invention is particularly useful for assembling connections of thetype shown in U.S. Pat. No. 3,359,013. This type of connection has oneor more annular shoulders associated with each thread, for engaging acorresponding shoulder on the mating piece. The threads themselves,being of a non-interference type, do not provide sealing, which occursentirely at the contacting shoulders. During assembly, the pipe can berotated by hand until shoulder contact occurs; thereafter only minorrotation, perhaps one-eighth turn, is needed to fully make up theconnection. During this stage the required torque rises rapidly fromhand-tight to, for example, 2000 ft. lbs. Comparative charts of torque Tvs. turns N for conventional and shouldered threads are shown in FIGS.3a and 3b. Plainly, the more rapid torque increase rate of theshouldered connection calls for a torque controller having fastresponse.

SUMMARY OF THE INVENTION

According to this invention, a shouldered pipe connection is made up intwo stages. During the initial stage, the joint is rotated at a speed ofabout 20 rpm at very low torque (up to about 50 ft.-lb.) until thesealing shoulders engage. Thereafter only minor additional rotation isneeded to seal the connection. During the final tightening stage, thepipe is rotated much more slowly up to a maximum torque limit on theorder .[.of.]. .Iadd.to .Iaddend.2000 ft. lb. Optimum rotation speedsand makeup torques may vary, depending upon type. Specifications areusually provided by the pipe manufacturer.

Torque is automatically controlled during both tightening stages. In theinitial stage, thread damage in the event of cross-threading isprevented by maintaining a very low torque cutoff point. In the finaltightening stage, galling and breaking of threads is prevented by slowlyturning the pipe 4 and automatically disabling the pipe tongs when apredetermined torque level is reached.

A primary object of the invention is to prevent overtorquing ofshouldered connections. Another object is to prevent thread damage inthe event of cross threading.

A further object is to protect the tongs operator from rapid tongsreaction movement when the tongs are initially actuated.

Another object is to enable the operator to control both the maximumobtainable tongs torque and the tongs speed during the final stage ofconnection makeup.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the invention diagrammatically.

FIG. 2 is a side elevation of a tongs unit forming part of theinvention.

FIGS. 3a and 3b show comparative torque charts for conventional (FIG.3a) and shouldered (FIG. 3b) connections.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the invention is illustrateddiagrammatically in FIG. 1. The major components are a conventionalhydraulic power unit A, a power tongs T driven by fluid from the powerunit, a tongs sensor/recorder B and a torque control module C.

The power unit A, as shown in FIG. 1, comprises an internal combustionengine 10, a hydraulic pump 12 driven thereby, a pressure regulator 14downstream of the pump, and a fluid reservoir 16 upstream of the pump.In operation, the power unit delivers pressurized fluid through highpressure line 20, and receives fluid exhausted by the tongs via returnline 22.

The tongs T have both conventional and novel aspects. A conventionalbody 30 supports rotary jaws 32 adapted to engage the outside diameterof a pipe P. The body houses a gear train, details of which are notshown, including a two- or multi-speed transmission. Tongs of this typeare well known. The transmission is manually shifted by means of a gearselector 34, with the ratio between high and low speeds being on theorder of 4:1. The tongs are powered by a hydraulic motor 36 drivingthrough two planetary gear reduction units 38 and 40 (FIG. 2) in series,each having about 51/2:1 reduction ratio. Further speed reduction isprovided by spur gearing within the tongs body, so that the overallreduction is about 60:1 in high gear and 250:1 in low gear.

The tongs motor 36 is driven by fluid from the power unit, which entersthe tongs via inlet line 42 and returns via exhaust line 44. A reversingshunt valve 46 on the tongs connected between the inlet and exhaustlines allows fluid to bypass the motor entirely when the valve is open.The shunt valve, normally open, may be moved to drive the tongs motor ineither direction by a manual throttle handle 50 accessible to operator.

Any torque applied to the pipe P by the tongs creates a reaction torquethat tends to rotate the tongs around the pipe. This tendency isrestrained by a snub line 54 connected between a stationary object andthe tongs body along a tangent line as shown. The snub line 54 includestwo load transducers in series for monitoring tongs torque. The firsttransducer 56 is an on-off pneumatic valve having adjustable springbias. This valve opens when tension corresponding to a preset"hand-tight" torque in the rage of 0-50 ft. lb. is applied. A manualoverride valve 58 in series with the first transducer 56 provides meansby which the operator can disable the hand-tight torque control system,if desired.

An important feature of the invention is the on-off valve 60mechanically connected via linkage 62 to the gear selector lever 34,such that the valve 60 is open only when the tongs are in theirhigh-speed range, as shown. As a result, the transducer 56 performs itstorque limiting function only during the initial, high speed phase.[.of.]. .Iadd.to .Iaddend.tongs operation, and does not interfere withhigh torque operation during the final stage of makeup.

The snub line 54 also has mounted therein a second load transducer 61which communicates via conduit 62 with a Bourdon tube 64 supportedwithin the recorder module B. The free end of the Bourdon tube isconnected to the stylus 65 of a conventional chart recorder 66 having aspring-driven motor 68. The stylus has a small blade 70 attached theretocapable of interrupting flow of air through a normally open air gap unit72, which can be moved toward or away from the stylus by means ofthreaded support 74 to adjust the threshold makeup torque. The air gapunit is supplied with air regulated to a very low pressure, e.g. 5 psi,so as not to affect stylus position. The output signal is amplified andinverted by the pneumatic logic unit 76, details of which are shown inapplicant's copending application Ser. No. 526,611, the disclosure ofwhich is incorporated by reference. The logic unit 76 thus generates ahigh pressure output in conduit 78--provided the second override valve80 is open--when the stylus blade 70 enters the air gap as the tongsreach maximum makeup torque. Conduit 78 leads to one input of a two-waycheck valve 82, the other input of which is from the hand-tighttransducer 56. A high pressure at either input is thus delivered viaconduit 84 to a second pneumatically actuated shunt valve 86, which whenactuated halts tongs operation.

The valve 60, first transducer 56 and shunt valve 86 together providemeans for halting tongs operation at a preset hand-tight torque level.Lever 34, linkage 62 and valve 60 function as means for disabling thisfirst means. This general terminology is used in the claims below. Thesecond transducer 61, recording module B and shunt valve 86 comprisemeans for halting tongs operation at a preset .[.fuel.]. .Iadd.full.Iaddend.makeup torque level.

Turning to the torque control module C, it can be seen that the tongsexhaust line 44 is directly connected to return line 22, while the tongsinlet line 42 is variably regulated as to both pressure and flow rate.Fluid entering the module from supply line 20 first encounters athree-way pneumatically actuated valve 88, whose position is ultimatelydetermined by the position of gear selector lever 34. In high gear,fluid is directed to line 90, which is regulated to very low pressure inthe range of 25-200 psi by the adjustable pressure regulator 92, whichrelieves excess pressure back to the return line 22.

When the tongs are in low gear, and valve 60 blocks delivery of controlpressure to valve 88, the supply line 20 is connected to a unregulatedhigh pressure line 94 having therein a manually adjustable flow ratecontroller 96. This valve enables the operator to control maximum tongsspeed during the final makeup stage, without affecting the maximumtorque obtainable. The variable restriction 98 shunting supply andreturn lines 20 and 22, on the other hand, enables the operator to limitthe pressure deliverable to the tongs. Maximum tongs torque can thus belimited, providing a measure of redundancy over the automatic controlsystem defined between transducer 61 and shunt valve 86.

In operation, as a drill string is supported by slips or the like on arig deck, a new joint is brought into mating contact with the next lowerjoint, Once the threads are engaged, the tongs operator, having placedthe gear selector in high, throws throttle 50, thereby closing shuntvalve 46 to apply regulated pressure from line 42 to the tongs motor,which rotates the pipe slowly at about twenty rpm hand tight. Note thatcompressed air passes through valve 60 to valve 88, which directs allhydraulic fluid flow past low pressure regulator 92, substantiallylimiting the torque capacity of the tongs. Furthermore, air pressure issupplied to first transducer 56. When the preset threshold snub lineload is reached, air passes through transducer 56, override valve 58 andcheck valve 82 to open the second shunt valve 86 and automatically stopthe tongs. In the event of improper thread engagement, this sequence ofevents disables the tongs before thread damage occurs, regardless of theoperator's attentiveness or reaction time, and corrective action can betaken. It is not necessary, with this system, to count turns of piperotation or the like.

Provided the connection is properly run up to hand tight, and theoperator can see that the sealing shoulders have come into contact, hethen places the gear selector lever in "low", automatically obstructingthe high pressure control signal to the second shunt valve 86, whichthereupon closes so that tongs operation can be resumed. Simultaneously,the valve 88 reverses position.[.,.]. so that fluid at full pressure isdelivered to the tongs. Now developing high torque, the tongs rotate thepipe very slowly--at five rpm or less, and this speed can be regulatedby means of valve 96--until the desired makeup torque is reached. At thepresent cutoff torque level, stylus blade 70 enters the air gap unit,causing logic unit 76 to deliver a high pressure signal to open thesecond shunt valve 86, thereby automatically halting tongs operation.

The embodiment of the invention described above has proven extremelyreliable in testing. The absence of sophisticated electronic monitors,alarms, and the like is attractive from a cost and repairabilitystandpoint, and in fact the torque record charts have demonstratedunequaled consistency from connection to connection.

The foregoing is a description of but one embodiment of the invention,whose full scope is described by the following claims. Variousmodifications within the scope of the invention may occur to those ofskill in the art. For example, electronic components could besubstituted for the pneumatic components described. A fully pneumaticsystem is presently preferred, however, because many rig operatorsunderstandably prefer to keep electrical devices of all types away fromthe rig deck.

I claim:
 1. A control system for a power tongs comprising a snub linefor restraining the tongs,a first snub line tension transducer forgenerating signals proportional to tongs torque in the hand-tight range,a second snub line tension transducer for generating signalsproportional to tongs torque in the full makeup torque range, firstmeans responsive to said first transducer for halting tongs operation ata preset hand-tight torque level, second means responsive to said secondtransducer for halting tongs operation at a preset full makeup torquelevel, and means for disabling said first means.
 2. The system describedin claim 1 wherein said tongs have high and low speeds and a gearselector for choosing between said speeds, andsaid disabling means iscontrolled by said gear selector.
 3. The system described in claim 2wherein said disabling means is actuated to disable said first meansonly when the tongs are operated in the lower of said two speeds.
 4. Thesystem described in claim 3 wherein the ratio between said high and lowspeeds is at least 4:1.
 5. The system described in claim 4 wherein theoverall gear ratio between the tongs motor and the tongs jaws is about250:1 in low speed and about 60:1 in high speed. .Iadd.
 6. A controlsystem for a power tongs comprisingmeans for restraining the tongs fromrotation, a first transducer operatively connected to said restrainingmeans for generating signals proportional to tongs torque in thehand-tight range, a second transducer operatively connected to saidrestraining means for generating signals proportional to tongs torque inthe full makeup torque range, first means responsive to said firsttransducer for halting tongs operation at a preset hand-tight torquelevel, second means responsive to said second transducer for haltingtongs operation at a preset full makeup torque level, and means fordisabling said first means..Iaddend.